1. Field of the Invention
The present invention generally relates to a radiation hardened charge pump for electronic circuitry, and in particular to the use of a polysilicon diode in combination with a capacitor, and to the use of multiple redundant charge pumps.
2. Background of the Invention
Integrated charge pump circuits have been in use since the 1970's. For commercial nonvolatile memories, the point of failure in radiation environments is usually the on-chip high voltage charge pump. For Total Ionizing Dose (TID) exposure the charge pumps fail due to parasitic metal oxide semiconductor (MOS) field leakage. For Single Event Effects (SEE), the charge pumps fail due to either destructive bipolar snap-back, Single Event induced Latch-Up (SEL), or Single Event Gate oxide Rupture (SEGR). Most of these effects are aggravated by on-chip high voltages, which are unique to non-volatile memories.
Phase-Locked-Loop (PLL) timing control circuits have a weak point in radiation environments. There is a voltage-reference-generation circuit in the feedback loop. The voltage reference is used to set the frequency of Voltage Controlled Oscillator (VCO). This reference generator is often known as a charge pump. It usually contains a current source and current sink to alternatively rise or lower the reference voltage. The control signals are typically labeled “up” and “down” respectively. The current source and sink are constructed of metal oxide semiconductor field effect transistors (MOSFETs). These MOSFETs are sensitive to Field leakage or Digital-Single-Event-Transients (DSETs) which can cause permanent or temporary failure of the behavior of the PLL.